Research opportunities with compact accelerator-driven neutron sources

An achromatic cold-neutron microscope with magnification 4 is demonstrated. The image-forming optics is composed of nested coaxial mirrors of full figures of revolution, so-called Wolter optics. The spatial resolution, field of view, and depth of focus are measured and found consistent with ray-tracing simulations. Methods of increasing the resolution and magnification are discussed, as well as the scientific case for the neutron microscope. In contrast to traditional pinhole-camera neutron imaging, the resolution of the microscope is determined by the mirrors rather than by the collimation of the beam, leading to possible dramatic improvements in the signal rate and resolution.

Demonstration of achromatic cold-neutron microscope utilizing axisymmetric focusing mirrors

Neutron guide tubes are widely used to transport neutron beams over long distances. The neutron mirrors used to line the guide tubes have imperfect reflectivity and, in long conventional guides, the average number of reflections for neutron rays becomes large thus reducing the transmission. This issue is extremely important for modern spallation sources, especially for the proposed long pulse European Spallation Source to be constructed in Lund, Sweden, where technical constraints require many instruments to be far from the source. Several solutions to the problem of transporting neutrons over long distances have been proposed and currently the most favored model is that of guides with elliptic shapes. It is widely believed that elliptic guides transport neutron rays from source to sample with a single bounce, a near perfect solution for long neutron guides, and a view which is true in ideal circumstances. This article uses computed Monte Carlo ray tracing simulations (VITESS) and other techniques to demonstrate that transport of neutrons by realistic elliptic guides usually involves many reflections, contrary to the usual expectations. These multiple reflections explain the irregular divergence distributions observed in computer simulations of transmission by some elliptic guides.

Multiple reflections in elliptic neutron guide tubes

Small-angle neutron scattering is an important technique for a number of fields, but most instruments are inefficient in terms of neutron flux Using a design based on axisymmetric focussing mirrors, Liu et al build a compact small-angle neutron-scattering setup that could overcome the present limitations

Demonstration of a novel focusing small-angle neutron scattering instrument equipped with axisymmetric mirrors

The cold neutron spectrum of the Beamline for neutron Optics and other Applications (BOA) at Paul Scherrer Institut enables high contrast neutron imaging because neutron cross sections for many materials increase with neutron wavelength. However, for many neutron imaging applications, spatial resolution can be as important as contrast. In this paper the neutron transmission imaging capabilities of an MCP/Timepix detector installed at the BOA beamline are presented, demonstrating the possibilities for studying sub-20 µm features in various samples. In addition to conventional neutron radiography and microtomography, the high degree of neutron polarization at the BOA beamline can be very attractive for imaging of magnetic fields, as demonstrated by our measurements. We also show that a collimated cold neutron beamline combined with a high resolution detector can produce image artifacts, (e.g. edge enhancements) due to neutron refraction and scattering. The results of our experiments indicate that the BOA beamline is a valuable addition to neutron imaging facilities, providing improved and sometimes unique capabilities for non-destructive studies with cold neutrons.

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High resolution neutron imaging capabilities at BOA beamline at Paul Scherrer Institut

We demonstrate neutron beam focusing by axisymmetric mirror systems based on a pair of mirrors consisting of a confocal ellipsoid and hyperboloid. Such a system, known as a Wolter mirror configuration, is commonly used in X-ray telescopes. The axisymmetric Wolter geometry allows nesting of several mirror pairs to increase collection efficiency. We implemented a system containing four nested Ni mirror pairs, which was tested by the focusing of a polychromatic neutron beam at the MIT Reactor. In addition, we have carried out extensive ray-tracing simulations of the mirrors and their performance in different situations. The major advantages of the Wolter mirrors are nesting for large angular collection and aberration-free performance. We discuss how these advantages can be utilized to benefit various neutron scattering methods, such as imaging, SANS, and time-of-flight spectroscopy.

Yelena S. Bagdasarova

Papers

From x-ray telescopes to neutron scattering: Using axisymmetric mirrors to focus a neutron beam

From x-ray telescopes to neutron scattering: Using axisymmetric mirrors to focus a neutron beam